1. Field of the Invention
The present invention relates to a method of making an improved carbon foam material and particularly a graphitized carbon foam material having superior compressive strength and electrical conductivity.
2. Description of the Prior Art
It has been known for many decades that coal can be beneficiated for application in a wide variety of environments. For example, it has been known that coal may be employed as a fuel in electric utility plants and, in respect of such usages, beneficiating of the coal will reduce the ash content and the amount of sulfur and nitrogen species contained in the gaseous exhaust products.
It has also been known to convert coal into coke for use in various process metallurgy environments.
It has also been known to create carbon foam materials from feedstocks other than coal, which can be glassy or vitreous in nature, and are brittle and not very strong. These products which lack compressive strength tend to be very brittle and are not graphitizable. See, generally, Wang, xe2x80x9cReticulated Vitreous Carbonxe2x80x94A New Versatile Electrode Material,xe2x80x9d Electrochimica Acta, Vol. 26, No. 12, pp. 1721-1726 (1981) and xe2x80x9cReticulated Vitreous Carbon An Exciting New Material,xe2x80x9d Undated Literature of ERG Energy Research and Generation, Inc. of Oakland, Calif.
It has been known through the analysis of mechanical properties of carbon fibers that long-range crystallite orientation is achieved by alignment of the precursor molecules during fiber spinning. In xe2x80x9cIdealized Ligament Formation in Geometry in Open-Cell Foamsxe2x80x9d by Hager et al., 21st Biennial Conference on Carbon, Conf. Proceedings, American Carbon Society, Buffalo, N.Y., pp. 102-103 (1993), a model analysis regarding interconnected ligament networks to create geometric evaluation of hypothetical ligamentous graphitic foam is disclosed. This model analysis, however, does not indicate that graphite foam was made or how to make the same.
It has been suggested to convert petroleum-derived mesophase pitch into a carbon foam product by employing a blowing/foaming agent to create bubbles in the material, followed by graphitization of the resultant carbonized foams above 2300xc2x0 C. See xe2x80x9cGraphitic Carbon Foams: Processing and Characterizationsxe2x80x9d by Mehta et al., 21st Biennial Conference on Carbon, Conf. Proceedings, American Carbon Society, Buffalo, N.Y., pp. 104-105 (1993). It is noted that one of the conclusions stated in this article is that the mechanical properties of the graphitic cellular structure were quite low when compared to model predictions.
It has been known to suggest the use of graphitic ligaments in an oriented structure in modeling related to structural materials. See xe2x80x9cGraphitic Foams as Potential Structural Materials,xe2x80x9d Hall et al., 21st Biennial Conference on Carbon, Conf. Proceedings, American Carbon Society, Buffalo, N.Y., pp. 100-101 (1993). Graphitic anisotropic foams, when evaluated mathematically in terms of bending and buckling properties, were said to have superior properties when compared with other materials in terms of weight with particular emphasis on plate structures. No discussion of compressive properties is provided.
In xe2x80x9cCarbon Aerogels and Xerogelsxe2x80x9d by Pekala et al., Mat. Res. Soc. Symp. Proc., Vol. 270, pp. 3-14 (1992), there are disclosed a number of methods of generating low-density carbon foams. Particular attention is directed toward producing carbon foams which have both low-density (less than 0.1 g/cc) and small cell size (less than 25 microns). This document focuses upon Sol-gel polymerization which produces organic-based aerogels that can be pyrolyzed into carbon aerogels.
In xe2x80x9cCarbon Fiber Applications,xe2x80x9d by Donnet et al., xe2x80x9cCarbon Fibers,xe2x80x9d Marcel Decker, Inc., pp. 222-261 (1984), mechanical and other physical properties of carbon fibers were evaluated. The benefits and detriments of anisotropic carbon fibers are discussed. On the negative side, are the brittleness, low-impact resistance and low-break extension, as well as a very small coefficient of linear expansion. This publication also discloses the use of carbon fibers in fabric form in order to provide the desired properties in more than one direction. The use of carbon fibers in various matrix materials is also discussed. A wide variety of end use environments, including aerospace, automotive, road and marine transport, sporting goods, aircraft brakes, as well as use in the chemical and nuclear industries and medical uses, such as in prostheses, are disclosed.
It has been known to make carbon fibers by a spinning process at elevated temperatures using precursor materials which may be polyacrylonitrile or mesophase pitch. This mesophase pitch is said to be achieved through conversion of coal-tar or petroleum pitch feedstock into the mesophase state through thermal treatment. This thermal treatment is followed by extrusion in a melt spinning process to form a fiber. The oriented fiber is then thermoset and carbonized. To make a usable product from the resulting fibers, they must be woven into a network, impregnated, coked and graphitized. This involves a multi-step, costly process. See xe2x80x9cMelt Spinning Pitch-Based Carbon Fibersxe2x80x9d by Edie et al., Carbon, Vol. 27, No. 5, pp. 647-655, Pergamen Press (1989).
There remains, therefore, a very substantial need for an improved method of making carbon foam product which has enhanced compressive strength and is graphitizable and the resultant products.
The present invention has met the above-described needs. In one preferred method of the present invention, a coke precursor is provided by de-ashing and hydrogenating bituminous coal. The hydrogenated coal is then dissolved in a suitable solvent which facilitates de-ashing of the coal and separation of the asphaltenes from the oil constituent. The asphaltenes are subjected to coking, preferably at a temperature of about 325xc2x0 C. to 500xc2x0 C. for about 10 minutes to 8 hours to devolatilize the precursor asphaltenes. The coking process is preferably effected at a pressure of about 15 to 15,000 psig. The anisotropic carbon foam so created is then cooled. In a preferred practice of the invention, the anisotropic carbon foam so created is subsequently graphitized. As an alternate to employing hydrogenation, solvent de-ashing of the raw coal alone may be employed in order to create the asphaltenes which are then coked and graphitized in the same manner. With this approach, an isotropic product is produced from the solvent extraction of raw unhydrogenated coal.
In a preferred practice of the invention, a blend of hydrogenated and unhydrogenated solvent separated asphaltenes may be employed in order to adjust the degree of anisotropy present in the carbon foam. Also, it is preferred that the voids in the foam may be generally of equal size. The size of the individual bubbles or voids may be adjusted by altering the amount of volatile material contained in the asphaltenes or varying the pressure under which coking is effected.
In a preferred practice of the invention, after coking, the foamed material is subjected to calcining at a temperature substantially higher than the coking temperature to remove residual volatile material. The preferred temperature is about 975xc2x0 C. to 1025xc2x0 C. and the time is that which is adequate to achieve a uniform body temperature for the material.
The method produces a graphitized carbon foam product having a compressive strength in excess of about 600 lb/in2.
It is an object of the present invention to provide a method of producing coal-derived carbon foam which may be graphitized.
It is a further object of the present invention to provide such a method and the resulting product which may be produced by hydrogenating bituminous coal followed by separation of asphaltenes, and coking the same.
It is a further object of the present invention to provide a method and resultant product which permits control of the degree of anisotropy of the carbon foam.
It is a further object of the present invention to provide such a method wherein solvent partitioning of the unhydrogenated coal or hydrogenated coal is employed to select the proper fraction for making the desired foam or removing inorganic species from the coal.
It is a further object of the present invention to provide such a method which permits control over the size of the voids in the carbon foam and the density of the same.
It is a further object of the invention to provide a method of producing such a product which is capable of being graphitized and has much higher compressive strength than previously known carbon foams.
It is a further object of the present invention to provide such a method which produces a controllable, low-density carbon foam product having either isotropic or anisotropic graphite structure which may have open-cell or close-cell configurations.
It is a further object of the present invention to provide a method of producing such a product which is lightweight and possesses a controllable degree of electrical and thermal conductivity.
These and other objects of the invention will be more fully understood from the following detailed description of the invention on reference to the illustrations appended hereto.